Melanin, a pigment found in human skin, is produced by cells known as melanocytes. Melanocytes produce melanin in granules called melanosomes. Melanosomes are transferred from the melanocytes to keratinocytes, a layer of keratin-producing cells that is closer to the outer surface of the skin. The more melanosomes that are transferred to the keratinocytes, the darker the skin appears. This process can be altered in persons of any skin type or race. Alteration that results in excessive darkening is known as hyperpigmentation.
Hyperpigmentation can take a variety of forms. Melasma is a form of hyperpigmentation in pregnant women that is characterized by dark patches on the cheeks and forehead, and is sometimes called "pregnancy mask". With age, many persons develop dark spots sometimes known as "liver spots." Hyperpigmentation is sometimes a side effect of birth control pills, and can be a persistent result of acne, bums, bites and other skin injuries.
The only treatment for hyperpigmentation that is approved in the United States for use by consumers without a prescription is the topical application of 1,4-benzenediol, also known as hydroquinone. It acts by suppressing melanocyte activity. Hydroquinone, by itself or in combination with glycolic acid, is sold without prescription at strengths of up to 2% and at strengths of up to 4% by prescription. Hydroquinone preparations are effective, but not without drawbacks. They can cause burning, redness, sensitization and irritation in some persons. Close supervision of the patient by a physician is recommended when prescription strength preparations are used. It is desirable to provide a treatment for hyperpigmentation that is at least as effective as hydroquinone, but lacks hydroquinone's side effects.